As illustrated in FIGS. 3A, 3B, 3C, it is possible to measure the damage length of an object to be measured that is made by adhering a member 1 and member 2. In FIGS. 3A, 3B, 3C, an oscillator 3 made from a piezo element or the like is placed on the member 1, and a vibration detection sensor 4 such as an FBG (Fiber Bragg Grating) optical fiber sensor is placed on the member 2. The object being measured is vibrated by the oscillator 3, and the arrival time of the vibration that propagates through the object being measured from the oscillator 3 to the vibration detection sensor 4 is measured. Measurement of the arrival time is first performed for an object having no damage as a calibration as illustrated in FIG. 3A. In this case, vibration (elastic wave) propagates along a vibration propagation path 11a as illustrated in FIG. 3A from the oscillator 3 to the vibration detection sensor 4. The arrival time of this vibration is recorded.
Next, as the actual measurement, the oscillator 3 and vibration detection sensor 4 are placed, as a rule, in the same positions on the object to be measured, which has the same construction as the object above, however for which it is unknown whether or not there is damage, as illustrated in FIGS. 3B and 3C, and the arrival time is measured (in the case that the oscillator 3 and vibration detection sensor 4 are not placed in the same position, the propagation time for that difference is subtracted). As illustrated in FIG. 3B, when there is damage in the form of peeling 12a occurring on an adhesion layer 10, vibration propagates along the vibration propagation path such as illustrated in the figure, however, the peeling 12 does not extend to underneath the vibration detection sensor 4, so there is hardly any effect on the arrival time of the vibration. However, as illustrated in FIG. 3C, when there is peeling 12b that extends to underneath the vibration detection sensor 4, vibration propagates along the vibration propagation path 11c as illustrated in the figure and goes around the peeling, so there is a delay in the arrival time of the vibration. Therefore, by calculating the difference between the arrival time during actual measurement and the arrival time during calibration, it is possible to determine whether or not peeling has extended to underneath the vibration detection sensor 4. Furthermore, when peeling has extended to underneath the vibration detection sensor 4, it is possible to calculate the peeling length as will be explained below.
As illustrated in FIGS. 3A to 3C, the distance from the end section of the adhesion layer 10 between oscillator 3 and the vibration detection sensor 4 to the oscillator 3 is taken to be “a”, and the distance to the vibration detection sensor 4 is taken to be “b”. The distances “a” and “b” are known values. When it is determined that peeling has extended to underneath the vibration detection sensor 4 due to the occurrence of a delay in the arrival time, the length of peeling that further extends from the vibration detection sensor 4 is taken to be distance “c” as illustrated in FIG. 3C. The peeling length to be found is (b+c). The delay of the arrival time is taken to be Δt. The difference between the propagation distance along the vibration propagation path 11a and the propagation distance along the vibration propagation path 11c is (2×c). Therefore, when the group velocity of vibration is taken to be V, Δt=(2×c)/V, and this equation can be transformed to c=(V×Δt)/2. Therefore, c can be found by entering the measured Δt into the equation above, and the peeling length is found as (b+c)=(b+(V×Δt)/2).
Analysis of the vibration detected by the vibration detection sensor 4 can be performed by a wave motion analysis apparatus that uses an optical filter such as disclosed in Japanese Patent Application Publication No. 2008-139171. By processing a detected wave motion signal, it is possible to acquire the change in time of the vibration as illustrated in FIGS. 4A and 4B, and to identify the peak of the vibration. By calculating the delay of the arrival time due to the shifting over time of the maximum peak, the damage length can be measured as described above.
In the case of a vibration waveform as illustrated in FIG. 4A, the reliability of the estimation of the damage length based on the arrival time Ta of the maximum peak p1 is high. However, depending on the state of the damage, a plurality of waves, such as reflected waves, overlap the vibration waveform as illustrated in FIG. 4B. Therefore, the maximum peak p3 occurs in a different location than the peak p2 that is originally supposed to be obtained, and the damage length estimated based on that arrival time Tb may be wrong. Conventionally, it could not be determined whether the maximum peak p3 was a peak due to the delay of the vibration as theoretically explained using FIGS. 3A to 3C, or was a peak that became high due to some other reason such as reflection. Therefore, there is a possibility that it will be determined that damage has occurred due to erroneous measurement.